In the last few years an increasing interest in implantable medical devices, such as cardiac stimulators, neurostimulators or systems enabling in vivo biomedical detection and control, has been observed. In the near future, implantable sensors will for example allow early diagnosis of certain diseases or allow the blood sugar or oxygen level to be precisely monitored in real time. However, in order to function autonomously, implantable medical devices require an internal power supply generally provided by a primary battery. Batteries, the energy densities of which are relatively high (as high as 300 Wh/l) allow small systems to be powered over a duration ranging from a few days to a few years depending on the required power. Thus, for implantable medical devices, the power densities of the highest-performance batteries currently available oblige the latter to be regularly replaced. However, for in vivo applications, the replacement of batteries is not a trivial matter because it requires a surgical intervention.
In order to avoid the replacement of primary batteries, it has been proposed to use what are called “secondary” batteries, recharged by way of an energy-harvesting device such as a coil (recharging by electromagnetic induction), an ultrasonic transducer (recharging by ultrasound) or alternatively a photovoltaic cell (recharging by near-infrared light). This type of device has the advantage of not requiring surgical interventions to replace discharged batteries. Specifically, provided the secondary batteries are periodically recharged by way of the aforementioned energy-harvesting devices, the lifetime of this type of device may approach that of the patient.
Photovoltaic recharging is a particularly promising approach, but one that is confronted with considerable technical difficulties.
In their article of 1999 “A wireless near-infrared energy system for medical implants” IEEE Engineering Medicine Biology 18, 70 (1999), K. Murakawa et al. proposed to use a near-infrared laser coupled to a photovoltaic cell made of silicon to power a secondary battery. It is a question of implanting the photovoltaic cell and the battery under the skin and illuminating the photovoltaic cell by way of the near-infrared laser, located outside the body, when the battery must be recharged. Specifically, the low absorption of near-infrared radiation (780-1400 nm) by biological tissues makes it possible for the radiation to reach the implanted photovoltaic cell. However, photovoltaic cells made of silicon have a relatively large thickness of about 100 μm, thereby making them stiff and therefore not very suitable for subcutaneous implantation. Thin-film photovoltaic cells, the thickness of which may be as small as 10 μm, and which may be produced on flexible substrates, are a more promising alternative. Among the various types of thin-film photovoltaic cells, those referred to as “CIGS” cells (CIGS is the acronym of copper-indium-gallium-selenium, these cells comprising an absorbing layer made of an alloy of general formula CuIn1-xGax(Se1-y,Sy)2, where 0<x<1, 0≦y≦1) are of particular interest because of their high efficiency—comparable to that of cells made of silicon and this for clearly much lower thicknesses.
However, CIGS cells have a characteristic that proves to be a major drawback in uses implementing transcutaneous illumination. Specifically, it has been demonstrated that such cells can achieve an optimal electrical performance only if the radiation illuminating them contains photons of energy higher than the width of the bandgap of their buffer layer. See in this regard the article by A. Pudov et al., “CIGS J-V distortion in the absence of blue photons”, Thin Solid Films 480 (2005), 273-278. Specifically, if the energy of the photons is higher than the width of the bandgap of the buffer layer, some of the photons are absorbed by the buffer layer creating electron-hole pairs, the photogenerated holes making it possible to neutralize acceptor type defects located on the surface of the absorber, thus decreasing the barrier to photoelectrons and enabling a more advantageous alignment of the bands.
Conventionally, the buffer layers used in this type of device are compounds based on zinc [ZnO1-xSx], cadmium (CdS) or indium (In2S3), the bandgap widths of which are 3.7 eV, 2.8 eV and 2.4 eV, respectively. Thus, it is necessary to illuminate cells having a buffer layer made of Zn(O1-xSx), In2S3 and CdS with photons of wavelength shorter than 335 nm (ultraviolet), 443 nm (blue) and 516 nm (green), respectively. Even though at wavelengths in the green the transmission of biological tissues is higher than at wavelengths in the blue or ultraviolet, the use of buffer layers based on Cd is not envisionable in implanted biomedical applications because of the very high toxicity of Cd.
It is therefore difficult to envision employing CIGS photovoltaic cells in implantable applications, since the photons in the ultra-violet or the blue that are necessary for the proper operation of these cells are absorbed by biological tissues.